Routers, also known as intermediate systems “IS”, are known, for example the Alcatel 7670 RSP router, which present operating discontinuities due to periods of unavailability. Periods of unavailability are due firstly to planned operations, such as router maintenance, and secondly to unexpected events, such as router failures. When other routers detect that a router has stopped or failed, their data concerning routing via the unavailable router is made invalid. When the router is put back into operation, all of that routing data is lost and needs to be exchanged anew with the other routers. This gives rise firstly to an increase in traffic on the network. It also gives rise to problems of accessibility on the network. Periods of router unavailability thus inconvenience users of the network.
A project for extending the IS—IS protocol has been proposed by the internet engineering task force (IETF) under the reference “Restart signaling for IS—IS” in order to enable programmed restarting of a router in a manner that reduces those drawbacks. However the corresponding protocol is neither available nor even tested. That project makes no provision for unplanned restarts. That project does not appear to be adequate for providing a sufficient increase in router availability. That project also requires changes to the standards concerning the IS—IS protocol. There is no prospect in the short term of changing the standards of the IS—IS routing protocol since that would require present routers to be updated or replaced.